ICE during codegen of function that can't be codegened

[vandenheuvel]

On stable 1.45.2, 1.47.0-beta.1 (2020-08-25 0f91f5c284c72e69bb71) and 1.47.0-nightly (2020-08-25 bf4342114e357f2934d5)

From the docstring of librustc_middle::mir::mono::MonoItem::is_instantiable:

pub fn foo<'a>(s: &'a mut ()) where &'a mut (): Clone {
    <&mut () as Clone>::clone(&s);
}

Possible related or duplicate issues are #69121 (doesn't seem to be as general) and #58375 (currently marked as solved).

Backtrace on stable 1.45.2

error: internal compiler error: src/librustc_trait_selection/traits/codegen/mod.rs:62: Encountered error `Unimplemented` selecting `Binder(<&mut () as std::clone::Clone>)` during codegen

thread 'rustc' panicked at 'Box<Any>', src/librustc_errors/lib.rs:907:9
stack backtrace:
   0: backtrace::backtrace::libunwind::trace
             at /cargo/registry/src/github.com-1ecc6299db9ec823/backtrace-0.3.46/src/backtrace/libunwind.rs:86
   1: backtrace::backtrace::trace_unsynchronized
             at /cargo/registry/src/github.com-1ecc6299db9ec823/backtrace-0.3.46/src/backtrace/mod.rs:66
   2: std::sys_common::backtrace::_print_fmt
             at src/libstd/sys_common/backtrace.rs:78
   3: <std::sys_common::backtrace::_print::DisplayBacktrace as core::fmt::Display>::fmt
             at src/libstd/sys_common/backtrace.rs:59
   4: core::fmt::write
             at src/libcore/fmt/mod.rs:1076
   5: std::io::Write::write_fmt
             at src/libstd/io/mod.rs:1537
   6: std::sys_common::backtrace::_print
             at src/libstd/sys_common/backtrace.rs:62
   7: std::sys_common::backtrace::print
             at src/libstd/sys_common/backtrace.rs:49
   8: std::panicking::default_hook::{{closure}}
             at src/libstd/panicking.rs:198
   9: std::panicking::default_hook
             at src/libstd/panicking.rs:218
  10: rustc_driver::report_ice
  11: std::panicking::rust_panic_with_hook
             at src/libstd/panicking.rs:490
  12: std::panicking::begin_panic
  13: rustc_errors::HandlerInner::bug
  14: rustc_errors::Handler::bug
  15: rustc_middle::util::bug::opt_span_bug_fmt::{{closure}}
  16: rustc_middle::ty::context::tls::with_opt::{{closure}}
  17: rustc_middle::ty::context::tls::with_opt
  18: rustc_middle::util::bug::opt_span_bug_fmt
  19: rustc_middle::util::bug::bug_fmt
  20: rustc_middle::ty::context::GlobalCtxt::enter_local
  21: rustc_trait_selection::traits::codegen::codegen_fulfill_obligation
  22: rustc_middle::ty::query::<impl rustc_query_system::query::config::QueryAccessors<rustc_middle::ty::context::TyCtxt> for rustc_middle::ty::query::queries::codegen_fulfill_obligation>::compute
  23: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
  24: rustc_data_structures::stack::ensure_sufficient_stack
  25: rustc_query_system::query::plumbing::get_query_impl
  26: rustc_ty::instance::resolve_instance
  27: rustc_middle::ty::query::<impl rustc_query_system::query::config::QueryAccessors<rustc_middle::ty::context::TyCtxt> for rustc_middle::ty::query::queries::resolve_instance>::compute
  28: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
  29: rustc_data_structures::stack::ensure_sufficient_stack
  30: rustc_query_system::query::plumbing::get_query_impl
  31: rustc_middle::ty::instance::Instance::resolve
  32: rustc_mir_build::lints::Search::is_recursive_call
  33: rustc_data_structures::graph::iterate::TriColorDepthFirstSearch<G>::run_from_start
  34: rustc_infer::infer::InferCtxtBuilder::enter
  35: rustc_mir_build::build::mir_built
  36: rustc_middle::ty::query::<impl rustc_query_system::query::config::QueryAccessors<rustc_middle::ty::context::TyCtxt> for rustc_middle::ty::query::queries::mir_built>::compute
  37: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
  38: rustc_data_structures::stack::ensure_sufficient_stack
  39: rustc_query_system::query::plumbing::get_query_impl
  40: rustc_mir::transform::check_unsafety::unsafety_check_result
  41: rustc_middle::ty::query::<impl rustc_query_system::query::config::QueryAccessors<rustc_middle::ty::context::TyCtxt> for rustc_middle::ty::query::queries::unsafety_check_result>::compute
  42: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
  43: rustc_data_structures::stack::ensure_sufficient_stack
  44: rustc_query_system::query::plumbing::get_query_impl
  45: rustc_mir::transform::mir_const
  46: rustc_middle::ty::query::<impl rustc_query_system::query::config::QueryAccessors<rustc_middle::ty::context::TyCtxt> for rustc_middle::ty::query::queries::mir_const>::compute
  47: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
  48: rustc_data_structures::stack::ensure_sufficient_stack
  49: rustc_query_system::query::plumbing::get_query_impl
  50: rustc_mir::transform::mir_validated
  51: rustc_middle::ty::query::<impl rustc_query_system::query::config::QueryAccessors<rustc_middle::ty::context::TyCtxt> for rustc_middle::ty::query::queries::mir_validated>::compute
  52: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
  53: rustc_data_structures::stack::ensure_sufficient_stack
  54: rustc_query_system::query::plumbing::get_query_impl
  55: rustc_mir::borrow_check::mir_borrowck
  56: rustc_middle::ty::query::<impl rustc_query_system::query::config::QueryAccessors<rustc_middle::ty::context::TyCtxt> for rustc_middle::ty::query::queries::mir_borrowck>::compute
  57: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
  58: rustc_data_structures::stack::ensure_sufficient_stack
  59: rustc_query_system::query::plumbing::get_query_impl
  60: rustc_query_system::query::plumbing::ensure_query_impl
  61: rustc_session::utils::<impl rustc_session::session::Session>::time
  62: rustc_interface::passes::analysis
  63: rustc_middle::ty::query::<impl rustc_query_system::query::config::QueryAccessors<rustc_middle::ty::context::TyCtxt> for rustc_middle::ty::query::queries::analysis>::compute
  64: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
  65: rustc_query_system::query::plumbing::get_query_impl
  66: rustc_middle::ty::context::tls::enter_global
  67: rustc_interface::interface::run_compiler_in_existing_thread_pool
  68: rustc_ast::attr::with_globals
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

note: the compiler unexpectedly panicked. this is a bug.

note: we would appreciate a bug report: https://github.com/rust-lang/rust/blob/master/CONTRIBUTING.md#bug-reports

note: rustc 1.45.2 (d3fb005a3 2020-07-31) running on x86_64-unknown-linux-gnu

note: compiler flags: -C embed-bitcode=no -C codegen-units=1 -C debuginfo=2 --crate-type lib

note: some of the compiler flags provided by cargo are hidden

query stack during panic:
#0 [codegen_fulfill_obligation] checking if `std::clone::Clone` fulfills its obligations
#1 [resolve_instance] resolving instance `<&mut () as std::clone::Clone>::clone`
#2 [mir_built] building MIR for `foo`
#3 [unsafety_check_result] unsafety-checking `foo`
#4 [mir_const] processing MIR for `foo`
#5 [mir_validated] processing `foo`
#6 [mir_borrowck] borrow-checking `foo`
#7 [analysis] running analysis passes on this crate
end of query stack
error: aborting due to previous error

Backtrace on 1.47.0-beta.1 (2020-08-25 0f91f5c)

error: internal compiler error: Encountered error `Unimplemented` selecting `Binder(<&mut () as std::clone::Clone>)` during codegen
  |
  = note: delayed at src/librustc_trait_selection/traits/codegen/mod.rs:65:32

thread 'rustc' panicked at 'no errors encountered even though `delay_span_bug` issued', src/librustc_errors/lib.rs:369:17
stack backtrace:
   0: std::panicking::begin_panic
   1: <rustc_errors::HandlerInner as core::ops::drop::Drop>::drop
   2: core::ptr::drop_in_place
   3: <alloc::rc::Rc<T> as core::ops::drop::Drop>::drop
   4: core::ptr::drop_in_place
   5: rustc_span::with_source_map
   6: rustc_interface::interface::create_compiler_and_run
   7: scoped_tls::ScopedKey<T>::set
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

error: internal compiler error: unexpected panic

note: the compiler unexpectedly panicked. this is a bug.

note: we would appreciate a bug report: https://github.com/rust-lang/rust/issues/new?labels=C-bug%2C+I-ICE%2C+T-compiler&template=ice.md

note: rustc 1.47.0-beta.1 (0f91f5c28 2020-08-25) running on x86_64-unknown-linux-gnu

note: compiler flags: -C embed-bitcode=no -C codegen-units=1 -C debuginfo=2 --crate-type lib

note: some of the compiler flags provided by cargo are hidden

query stack during panic:
end of query stack

Backtrace on 1.47.0-nightly (2020-08-25 bf43421)

error: internal compiler error: Encountered error `Unimplemented` selecting `Binder(<&mut () as std::clone::Clone>)` during codegen
  |
  = note: delayed at src/librustc_trait_selection/traits/codegen/mod.rs:65:32

thread 'rustc' panicked at 'no errors encountered even though `delay_span_bug` issued', src/librustc_errors/lib.rs:369:17
stack backtrace:
   0: std::panicking::begin_panic
   1: <rustc_errors::HandlerInner as core::ops::drop::Drop>::drop
   2: core::ptr::drop_in_place
   3: <alloc::rc::Rc<T> as core::ops::drop::Drop>::drop
   4: core::ptr::drop_in_place
   5: rustc_span::with_source_map
   6: rustc_interface::interface::create_compiler_and_run
   7: scoped_tls::ScopedKey<T>::set
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

error: internal compiler error: unexpected panic

note: the compiler unexpectedly panicked. this is a bug.

note: we would appreciate a bug report: https://github.com/rust-lang/rust/issues/new?labels=C-bug%2C+I-ICE%2C+T-compiler&template=ice.md

note: rustc 1.47.0-nightly (bf4342114 2020-08-25) running on x86_64-unknown-linux-gnu

note: compiler flags: -C embed-bitcode=no -C codegen-units=1 -C debuginfo=2 --crate-type lib

note: some of the compiler flags provided by cargo are hidden

query stack during panic:
end of query stack

[vandenheuvel]

Note that the function doesn't need to be public. Also, after reading the docstring that this example came from, it seems like an attempt is made to generate code, although no code should be emitted (the function can never be called, as the where clause can not be satisfied).

That function can't be codegened, because the method <&mut () as Clone>::clone does not exist. Luckily for us, that function can't ever be used, because that would require for &'a mut (): Clone to hold, so we can just not emit any code, or even a linker reference for it.

[LeSeulArtichaut]

This regressed in 1.44.0.

[apiraino]

@rustbot ping cleanup-crew
This issue could use a bisection to nail down where it starts. I think it regressed in between 1.43..1.44

[rustbot]

Hey Cleanup Crew ICE-breakers! This bug has been identified as a good
"Cleanup ICE-breaking candidate". In case it's useful, here are some
instructions for tackling these sorts of bugs. Maybe take a look?
Thanks! <3

cc @AminArria @camelid @chrissimpkins @contrun @DutchGhost @elshize @ethanboxx @h-michael @HallerPatrick @hdhoang @hellow554 @imtsuki @kanru @KarlK90 @LeSeulArtichaut @MAdrianMattocks @matheus-consoli @mental32 @nmccarty @Noah-Kennedy @pard68 @PeytonT @pierreN @Redblueflame @RobbieClarken @RobertoSnap @robjtede @SarthakSingh31 @senden9 @shekohex @sinato @spastorino @turboladen @woshilapin @yerke

[smmalis37]

---

Regression in dae90c1

---

searched nightlies: from nightly-2020-04-21 to nightly-2020-09-02
regressed nightly: nightly-2020-05-03
searched commits: from 7f65393 to f05a524
regressed commit: dae90c1

[spastorino]

Assigning P-medium as discussed as part of the Prioritization Working Group procedure and removing I-prioritize.